The size of an organ or organism depends mainly on the sizes and numbers of the cells it contains. In the first segment of my talk, I describe our work on cell size control in cultures of purified rat Schwann cells. Most proliferating cells grow before they divide, but it is not known how growth and division are coordinated to ensure that cells divide at an appropriate size. We have found that extracellular signals can control cell growth and cell-cycle progression separately and that the...